Nanoindenter

ABSTRACT

A new type of indenter is described. This device combines certain sensing and structural elements of atomic force microscopy with a module designed for the use of indentation probes, conventional diamond and otherwise, as well as unconventional designs, to produce high resolution and otherwise superior indentation measurements.

REFERENCES CITED

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BACKGROUND OF THE INVENTION

An AFM is a device used to produce images of surface topography (andother sample characteristics) based on information obtained fromrastering a sharp probe on the end of a cantilever relative to thesurface of the sample. Deflections of the cantilever, or changes in itsoscillation, which are detected while rastering correspond totopographical (or other) features of the sample. Deflections or changesin oscillation are typically detected by an optical lever arrangementwhereby a light beam is directed onto a cantilever in the same referenceframe as the optical lever. The beam reflected from the cantilever ismade to illuminate a position sensitive detector (PSD). As thedeflection or oscillation of the cantilever changes, the position of thereflected spot on the PSD changes, causing a change in the output fromthe PSD. Changes in the deflection or oscillation of the cantilever aretypically made to trigger a change in the vertical position of thecantilever base relative to the sample, in order to maintain thedeflection or oscillation at a constant pre-set value. It is thisfeedback that generates an AFM image. AFMs can be operated in a numberof different imaging modes, including contact mode where the tip of thecantilever is in constant contact with the sample surface, andoscillatory modes where the tip makes no contact or only intermittentcontact with the surface.

Actuators are commonly used in AFMs, for example to raster the proberelative to the sample surface or to change the position of thecantilever base relative to the sample surface. The purpose of actuatorsis to provide relative movement between the probe and the sample. Fordifferent purposes and different results, it may be useful to actuatethe sample, or the tip or some combination of both. Sensors are alsocommonly used in AFMs. They are used to detect movement of variouscomponents of the AFM, including movement created by actuators. For thepurposes of the specification, unless otherwise specified, the term“actuator” refers to a broad array of devices that convert input signalsinto physical motion, including piezo activated flexures, piezo tubes,piezo stacks, blocks, bimorphs, unimorphs, linear motors,electrostrictive actuators, electrostatic motors, capacitive motors,voice coil actuators and magnetostrictive actuators, and the term“position sensor” or “sensor” refers to a device that converts adisplacement, velocity or acceleration into an electrical signal,including capacitive sensors, inductive sensors (including eddy currentsensors), differential transformers (such as described in co-pendingapplications US20020175677A1 and US20040075428A1, Linear VariableDifferential Transformers for High Precision Position Measurements, andUS20040056653A1, Linear Variable Differential Transformer with DigitalElectronics, which are hereby incorporated by reference in theirentirety), variable inductance, optical interferometry, opticaldeflection detectors (including those referred to above as a PSD andthose described in co-pending applications US20030209060A1 andUS20040079142A1, Apparatus and Method for Isolating and MeasuringMovement in Metrology Apparatus, which are hereby incorporated byreference in their entirety), strain gages, piezo sensors,magnetostrictive and electrostrictive sensors.

SUMMARY OF THE INVENTION

We have developed a nanoindenter that produces very accurate,quantitative characterization for a wide spectrum of materials. The newnanoindenter may be implemented on an atomic force microscope (AFM)platform, but unlike indentation that might be effected with an AFMcantilever, the invention drives the indenting tip perpendicularly intothe sample. Displacement and force are measured with optimized LVDTsensors and an Optical Lever, respectively, the same devices thateliminate inaccuracies (e.g. non linearity) present in measurements madewith AFMs, and this greatly increases sensitivity and resolution incomparison to commercial indenters. This highly quantitative tool,incorporating high end AFM capabilities breaks new ground incharacterization of a great diversity of materials including thin films,coatings, polymers, etc. As noted, the nanoindenter may be implementedon an AFM platform and when integrated with the native metrologyabilities of the Molecular Force Probe-3D AFM of Asylum ResearchCorporation, it enables the user to perform quantitative indentationmeasurements and to make quantitative statements regarding the indentertip shape and indentation volumes and profiles, all with the sameinstrument set-up.

In addition to an AFM platform, the new nanoindenter may be implementedon other cantilever-based instruments. Cantilever-based instrumentsinclude such instruments as AFMs, molecular force probe instruments (1 Dor 3D), high-resolution profilometers and chemical or biological sensingprobes. For the sake of convenience, the specification focuses on AFMs.However, it should be understood that problems addressed and solutionspresented by the present invention are also applicable to otherinstruments with nanoscale motion and metrology applications.

The systems and techniques described herein provide a novel device fornanoscale metrology that permits quantitative measurements ofindentation and related parameters better than is presently possiblewith commercially available tools.

Specifics of the invention will be described in greater detail below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is the new indenter module attached to an existing AFM.

FIG. 2 is the new indenter module placed in the MFP-3D AFM of AsylumResearch Corporation.

FIG. 3 is the new indenter module prior to assembly.

FIG. 4 is the flexure mounted on the circular plate/retaining ring ofthe new indenter module.

FIG. 5 is the flexure of the new indenter module.

FIG. 6 is a cross sectional view of the mechanical converter system ofthe new indenter module.

FIG. 7 is a perspective view from the top of the mechanical convertersystem of the new indenter module.

FIG. 8 is another new indenter module prior to assembly.

FIG. 9 is a bottom-side.plan view of the indenter module of FIG. 8 afterassembly.

FIG. 10 is a plan view of the planar leaf springs of the assembledflexure of indenter module of FIG. 8.

FIG. 10 through FIG. 15 are plan views of planar leaf springs of otherassembled flexures for other indenter modules.

FIG. 16 is another new indenter module prior to assembly.

FIG. 17 is a bottom-side plan view of the indenter module of FIG. 16after assembly.

FIG. 18 is a plan view of the planar leaf springs of the assembledflexure of indenter module of FIG. 16.

FIG. 19 is a perspective view of the an optical lever detection systemdesigned to substitute for the mechanical converter system depicted inFIG. 6.

DETAILED DESCRIPTION

One preferred embodiment of the current invention is depicted in FIG. 1,showing a cross-sectional view of a module embodying the invention whichis installed in an AFM in place of the cantilever holder. Panels (A) and(B) of FIG. 1 each show the module rotated 90 degrees about the verticalcentral shaft of the module which culminates in the indenter probe 303positioned over a stage 307 on which a sample could be attached. Thisembodiment of the invention allows it to take direct advantage of someor all of the existing sensors and structures of an AFM.

FIG. 2 shows the module embodying the invention 12 installed in aMolecular Force Probe-3D AFM from Asylum Research Corporation (only thehead 13 of the MFP-3D is shown). In the configuration depicted in FIG.2, the indenter module 12 is removable and make use of the actuators,sensors and optics of the head 13, In particular, the z-piezo 14 of theMFP-3D is used to actuate the indenter probe 303 and the z-LVDT sensor17 of the MFP-3D may be used to measure the displacement of the indenterprobe 303. The position sensitive detector 16 of the MFP-3D is used tomeasure the motion of the flexure controlling the displacement of theindenter probe 303. Using data from these sensors, it is possible toquantify the displacement and force acting between the indenter probe303 and a sample (not shown).

Another useful feature of the indenter module 12 being installed in theMFP-3D head 13 is that the module can use certain optical features ofthe head for providing an optical view of the indenter probe 303 and thesample. The top-view objective 18 and steering mirror 19 of the MFP-3Dhead 13 work with the prism 6 of the indenter module 12 to provide anoptical view of both the indenter probe 303 and the sample (not shown),as well as to illuminate both. This is of great utility for aligning theindenter probe 303 with particular structures on the sample.

The indenter can similarly be installed in AFMs manufactured by othercompanies and, with modifications, in yet other AFMs. The MFP-3D ofAsylum Research Corporation is particularly amenable to conversion to anindenter because the actuators, sensors and optics located in its headare appropriate and convenient for indenter purposes. The same is trueof other AFMs, or could be made to be true with modifications.

Except where clearly stated otherwise, the remainder of this DetailedDiscussion of the current invention is directed at an indenter modulethat might be installed in any AFM, not just the MFP-3D of AsylumResearch Corporation, as well as to a stand-alone indenter that is not acomponent of an AFM.

FIG. 3 shows a perspective view of the module embodying the inventionprior to assembly. The indenter probe 303 is mounted on a removablechuck 304. The chuck 304 allows the user to employ a variety of standardand custom indenter probes and to change easily from one probe toanother. As is well-known to those versed in the art, these probes areformed of different materials, including diamond, tungsten, siliconnitride and others. The chuck 304 is attached to a monolithicthree-dimensional leaf spring flexure 305 which is designed to constrainthe motion of the indenter probe 303 to the z-axis only, that is,perpendicular to the sample. Precluding motion in the other axes is amajor contributor to the results available with the invention. Theflexure 305 is rigidly attached to the bottom of a circular plate with aretaining ring on its top 310 which is designed to mount into an AFM inplace of the cantilever holder. This permits the actuator (not shown),which in an AFM would be used to change the position of the cantileverbase in response to changes in the deflection or oscillation of thecantilever, to be used to actuate the circular plate/retaining ring 310and through it to actuate the flexure 305, the chuck 304 and theindenter probe 303. Measurement of this actuation may be improvedthrough the use of a sensor (not shown) to measure the displacement.

As shown in FIG. 3, the module embodying the invention may also includea prism 306 to permit an oblique view of the sample (not shown) on thesample holder 307, the indenter probe 303 and their interface where theAFM in which the module is mounted includes an optical view system. Themodule may also include a dust cover 308 to protect the flexure 305, thecircular plate/retaining ring 310 and the prism 306 from externalcontamination and prevent damage from handling. The dust cover 308 mayalso provide mechanical hard stops (not shown) to prevent the flexure305 from being overextended. A shake piezo (not shown) to oscillate theflexure 305, and thus the chuck 304 and the indenter probe 303, when themeasurement is to be made in an AC mode could be added at the pointwhere collar 320 screws into the flexure 305 or alternatively could beattached to the circular plate/retaining ring 310.

FIG. 4 is a perspective view of the flexure 305 mounted on the circularplate/retaining ring 310, with the chuck 304, the indenter probe 303 andthe prism 306 shown assembled. The flexure 305 is a monolithic,three-dimensional leaf spring with three supporting ends 403 and aflexing portion, the central shaft 404, which is free to move inresponse to the application of force. The supporting ends 403 are eachrigidly attached to the circular plate/retaining ring 310. The chuck 304is rigidly attached to one end of the flexing portion, the central shaft404, and the indenter probe 303 is attached to the chuck 304, facing thesample to be indented.

FIG. 5 is a second perspective view of the flexure 305, absent the otheritems shown in FIG. 4. Such flexures are fabricated using metals andmachining methods, including electronic discharge machining, well-knownto those versed in the art.

FIG. 6 shows a cross-sectional view of the central shaft 404 of theflexure 305, with the chuck 304 attached to one end (and the indenterprobe 303 attached to the chuck 304) and a collar 320 attached to theother end which is inserted through a hole in the center of the circularplate/retaining ring 310 and gives access to the central shaft 404 fromwithin the AFM to enable a mechanical converter assembly. The purpose ofthe mechanical converter assembly is to convert the z-axis linear motionof the central shaft 404, chuck 304 and indenter probe 303 into anangular change that an optical lever detector system, which in an AFMwould be used to detect changes in the deflection or oscillation of thecantilever, can measure. When the indenter module is mounted into an AFMin place of the cantilever holder and the circular plate/retaining ring310 is actuated, thereby displacing flexure 305, chuck 304 and indenterprobe 303 toward the sample, one end of a second flexure, planar flexure314, which is rigidly attached to the circular plate/retaining ring 310is also displaced. At the same time, the other end of planar flexure314, which on one side is linked though a ball bearing mechanism 318 tothe central shaft 404 and on the other side is attached to awedge-shaped mirror mount 315, and is free to flex is also displace,with the displacement following that of the central shaft 404. Thisdisplacement of the flexing end of the planar flexure 314 tilts themirror mount 315, and mirror 309 attached to the mount, and therebysteers the light beam of an optical lever detection system (not shown)from one place on the position sensitive detector of the system toanother thereby providing a measure of the displacement of the indenterprobe 303. A small permanent magnet 324 keeps the mirror mount 315 andplanar flexure 314 in contact with the ball bearing surface 318.

The design of the mechanical converter assembly allows the motion of theindenter tip 303 to be measured in much the same manner as cantilevertip deflection or oscillation are measured in a conventional AFM.Together with the use of the z-axis actuator previously discussed, thisallows the indenter module to be easily swapped with the cantileverholder of an AFM, resulting in a unique, useful and versatile instrumentthat allows the user to bring the functionality of an AFM to bear onindenting.

FIG. 7 shows a perspective view from above of the mechanical converterassembly discussed in the preceding paragraph. The two ends of theplanar flexure 314, the support end 316 rigidly attached to the circularplate/retaining ring 310, and therefore displaced in tandem withactuation of the circular plate/retaining ring, 310 and the flexing end317 linked though a ball bearing mechanism 318 to the central shaft 404and also attached to the mirror mount 315, and therefore displaced intandem with the central shaft, are displayed with greater clarity. FIG.7 also shows the hard stops 311 built into the mechanical converterassembly to prevent the flexure 305 from being overextended or damagedby normal handling.

The planar flexure 314 has a very high compliance when compared to theindenting flexure 305, that is, the spring constant of the planarflexure 314 is very low compared to the spring constant of the indentingflexure 305.

It will be observed that the indenting flexure 305 plays a major role inthe performance of the preferred embodiment of the current inventionwhich has just been discussed. A second preferred embodiment, employinga different indenting flexure but, like the first preferred embodiment,designed to be installed in an AFM in place of the cantilever holder inorder to make use of the actuators, sensors and optics of the AFM isdepicted in FIG. 8, a perspective view of the module embodying theinvention prior to assembly.

Unlike the monolithic three-dimensional leaf spring flexure 305 of thefirst preferred embodiment, the flexure of FIG. 8 is an assembledflexure. Like that flexure, however, the assembled flexure of FIG. 8 isdesigned to constrain the motion of the indenter probe 303 to the z-axisonly, that is, perpendicular to the sample. The flexing portion of theassembled flexure of FIG. 8 is provided by the central shaft of twocircular, planar leaf springs 506 and 507, which are firmly constrainedat the support end, that is the perimeter, by interleaved clamps, 503,504 and 505. The assembly of planar leaf springs 506 and 507 andinterleaved clamps, 503, 504 and 505 is rigidly attached to the circularplate/retaining ring 310. The central shaft of the planar leaf springs506 and 507 is composed of spindles 508 and 509. The upper end ofspindle 508 screws into the lower end of spindle 509, clamping planarleaf spring 506 against the stop at the lower end of spindle 509. Thestop at the upper end of spindle 509 is fastened to planar leaf spring507 and the portion of the spindle above the stop extends through a holein the center of the circular plate/retaining ring 310 and gives accessto the central shaft from within the AFM to enable the mechanicalconverter assembly discussed above in connection with the firstpreferred embodiment to function.

As with the first preferred embodiment, the central shaft of theassembled flexure of FIG. 8 ends with the chuck 304, rigidly attached tothe lower end of spindle 508, and the indenter probe 303 attached to thechuck 304, facing the sample to be indented. The assembled flexure, thechuck 304 and the indenter probe 303 are actuated by actuating thecircular plate/retaining ring 310 with the actuator (not shown) which inthe AFM would be used to change the position of the cantilever base inresponse to changes in the deflection or oscillation of the cantilever.

FIG. 9 is a bottom-side plan view of the flexure of FIG. 8 afterassembly. FIG. 10 is a plan view of the planar leaf springs 506 and 507of the flexure of FIG. 8. Such springs are fabricated using metals,including beryllium copper, and machining methods well-known to thoseversed in the art.

Other preferred embodiments of the current invention, also designed tobe installed in an AFM in place of the cantilever holder in order tomake use of the actuators, sensors and optics of the AFM, but employinga different indenting flexures than indenting flexure 305 of the firstpreferred embodiment or the assembled flexure of FIG. 8, form part ofthe invention. FIG. 11 through FIG. 15 are plan views of the planar leafsprings of such other preferred embodiments. Each of them consists of aflexing portion in the center of the spring, support ends at theperimeter and connections from the flexing portions to the support endsof variously shaped beams. In addition to the circles and other depictedshapes of these planar leaf springs, any polygon would in generalproduce similar results when paired with appropriate connections.

An additional preferred embodiment, again employing a differentindenting flexure but, like the other preferred embodiments disclosedabove, designed to be installed in an AFM in place of the cantileverholder in order to make use of the actuators, sensors and optics of theAFM is depicted in FIG. 16, a perspective view of the module embodyingthe invention prior to assembly. Like the flexure of FIG. 8, the flexureof FIG. 16 is an assembled flexure and is designed to constrain themotion of the corresponding probe 703 (which is also an optical element)to the z-axis only, that is perpendicular to the sample.

The components of the assembled flexure of FIG. 16 are similar to thoseof the assembled flexure of FIG. 8: two planar leaf springs 707 and 708separated by clamps 705 and 706. However, the assembled flexure of FIG.16 does not have a third clamp separating the upper planar leaf spring708 from the plate 709 which facilitates installation of the assembly inan AFM in place of the cantilever holder. Instead, planar leaf springs708 is rigidly attached directly to the plate 709.

The central shaft of the planar leaf springs 707 and 708 of theassembled flexure of FIG. 16 also differs from the central shaft of theassembled flexure of FIG. 8. The central shaft of the planar leafsprings 707 and 708 consists of just a chuck 704, and the probe 703attached to the chuck 704, facing the sample to be indented. The upperend of the chuck 704 extends through a hole in the center of the plate709 and gives access to the central shaft from within the AFM to enablethe mechanical converter assembly discussed above in connection with thefirst preferred embodiment to function.

The probe 703 of the assembled flexure of FIG. 16 is a new device notcurrently known in the art and as such is significantly different fromthe indenter probe 303 of the assembled flexure of FIG. 8. Probe 703 hasboth a different function than indenter probe 303 and a differentconstruction. Probe 703 has a dual function: it is both an opticalelement and an indenting probe. As an optical element, probe 703 acts asa lens allowing the optical viewer system of the AFM head in whichassembled flexure of FIG. 16 is installed direct optical view of thesample to be indented. As an indenting probe, probe 703 is a rigidelement that applies force and indents the sample as with traditionalindenter probes, such as 303. With this dual function comes arequirement for a different construction. A probe like probe 703 must befabricated from a material which is both very hard and transparent tolight. Suitable materials include diamond and sapphire.

Probe 703 has many advantages over other indenter transducers orindenting probes. These include making available true top optical viewof the sample to be indented, allowing for accurate positioning ofindentations and having a combination of low mass and high resonantfrequency, which allows unprecedented resolution in the measurement ofmechanical behavior of materials through both static and dynamicmaterial testing methods.

FIG. 17 is a bottom-side plan view of the flexure of FIG. 16 afterassembly. FIG. 18 is a plan view of the planar leaf springs 707 and 708of the flexure of FIG. 16. Such springs are fabricated using metals,including beryllium copper, and machining methods well-known to thoseversed in the art.

FIG. 19 shows an optical lever detection system designed to free themodules embodying the current invention from the use of the mechanicalconverter assembly described above in order to access the optical leverdetection system of an AFM. A mirror 1901 is attached to the collar 320which is rigidly attached to central shaft 404, chuck 304 and indenterprobe 303 of the flexure of the first preferred embodiment of theinvention. Mirror 1901 deflects incident light beam coming an opticallever detection system and as this mirror moves relative to the indenterprobe 303, PSD of the optical lever detection system measures thedisplacement. An additional lens 1902 may be placed above mirror 1901 toincrease sensitivity and facilitate calibration of the optical leverdetection system.

The described embodiments of the invention are only considered to bepreferred and illustrative of the inventive concept. The scope of theinvention is not to be restricted to such embodiments. Various andnumerous other arrangements may be devised by one skilled in the artwithout departing from the spirit and scope of the invention.

1. A nanoindenter, comprising a module installed in an atomic forcemicroscope which includes an actuator that may be used to actuate theindenter probe and an optical lever detection system that may be used tomeasure the motion of the flexure controlling the indenter probe; anindenter probe attached to the central shaft of a monolithicthree-dimensional leaf spring flexure which constrains the motion ofsuch probe to the axis perpendicular to the sample; and a mechanicalconverter assembly which converts the z-axis motion of such centralshaft to an angular change which may be measured by such optical leverdetection system.